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COSEWIC assessment and status report on the Atlantic Salmon (Inner Bay of Fundy populations) in Canada
- Assessment Summary
- Executive Summary
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status Designations
- Technical Summary
- Acknowledgements and Authorities Contacted
- Information Sources
- Biographical Summary of Report Writers
- Appendix 1. General biology of Atlantic salmon
Appendix 1. General biology of Atlantic salmon
The life cycle of the Atlantic salmon contains many stages. Canadian Atlantic salmon typically spawn in October and November, usually earlier in the north and later in the south. The timing of river entry varies among populations as an adaptation to local conditions and a response to water levels. In general, adults move into estuaries and then into freshwater rivers in the summer. During the physiological transformation for spawning, colours develop and the male grows a kype that is used in male-male fighting. The nest site is chosen by the female, typically in a gravel-bottom riffle above or below a pool, where she digs a nest pit. Males compete with each other for proximity to the female, and the dominant male and perhaps others release milt as she releases a portion of her eggs. Small, precociously mature parr may sneak into the nest and also release milt (Fleming and Reynolds 2004). The female covers the embryos with gravel and then digs another nest, repeating this process until 5-10 nests are made in an area termed a ‘redd’. The eggs, numbering from 3,000 to 4,000 per smaller female and increasing with body size, are large (5-7 mm) and contain a considerable quantity of yolk. At the end of the spawning season, surviving adults are termed ‘kelts’ and re-enter the ocean where they continue to grow until the next spawning season. Female kelting rates are usually higher than those for males. The eggs develop in the nest during the winter and, depending upon temperature, usually hatch in April. The young remain buried in the gravel as ‘alevins’, absorbing the yolk sac until May or June. The juveniles, termed ‘parr’, occupy rifles where they feed on invertebrate drift. After several years of freshwater growth, perhaps at 127-152 mm in length, the parr change physiologically into ‘smolt’ and migrate to the ocean. Growth in the ocean is rapid, and individuals may mature after one sea-winter as ‘grilse’ or after two or more sea-winters as ‘salmon’. Salmon feed on a variety of prey including crustaceans and small fish. Studies of adult sex ratio suggest that it varies around 1:1.
Migration and Dispersal
Atlantic salmon from eastern Canada typically migrate to feeding grounds near western Greenland. At maturity, they home to their river of origin. Straying rates are low, and typically less than 5% of the adult population will enter non-natal rivers. This extreme homing greatly reduces but does not prevent the potential for recolonization. Local adaptation in both adult breeders and their offspring can, however, greatly decrease the relative fitness of strays.
Adaptation and Adaptability
Atlantic salmon are locally adapted to almost all aspects of their life, including: adult body shape (depending on migration distance), run timing (depending on stream size and temperature), adult breeding phenotype (depending on density), adult age/size at maturity, egg size (depending on gravel size and juvenile competition), parr colouration (depending on background and crypticity), migration orientation, and so forth (e.g., Hendry and Stearns 2004). Nevertheless, the Atlantic salmon should not be thought to be a highly ‘adaptable’ species. The introduction of salmon across geographic ranges shows that introduced individuals have very low success, possibly because of the high degree of adaptation to their prior local conditions. Atlantic salmon have become domesticated within a few decades in aquaculture, but only through intensive selection in a few regions of the world (e.g., Norway, Scotland, Canada). There has been a long history of interest in stocking wild Atlantic salmon, and populations are now found in 20-30 regions outside their native distribution (MacCrimmon and Gots 1979). Conversely, there has been relatively little success in establishing or maintaining populations through hatchery supplementation within their native range (National Research Council 2004). Thus, the local adaptations of salmon should be considered difficult to replace once lost.
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